The present invention relates to a process for producing an .alpha.-olefin polymer and more particularly to a process for producing an .alpha.-olefin polymer in the presence of a novel catalyst system having a very high catalytic activity per a unit weight of solid catalyst component as well as per a unit gram atom of titanium. The produced .alpha.-olefin polymer contains extremely-decreased amounts of catalyst residue and of amorphous polymer and is superior in mechanical properties and processability.
It is well known to use generally the so-called Ziegler-Natta catalyst that comprises a transition metal compound, said metal being selected from the metals belonging to the group IV-VI of the periodic table, and an organometallic compound, said metal being selected from the metals belonging to the group I-III of the periodic table, in processes for producing polymers of .alpha.-olefins including propylene and butene-1.
In particular, titanium trichloride catalysts are used extensively for the industrial productions of .alpha.-olefin polymers.
However, any of these processes produces incidentally an amorphous polymer besides a high-stereoregular .alpha.-olefin polymer of high industrial utility value.
The amorphous polymer is limited in industrial utility value and has adverse effects on mechanical properties of the films, fibers, and other processed articles of the .alpha.-olefin polymer when contained therein.
In addition, the formation of the amorphous polymer causes a loss of raw material monomer and simultaneously makes a facility indispensable which is necessary to remove the amorphous polymer, thus bringing about very great disadvantages from the industrial point of view.
Accordingly, if such an amorphous polymer is not formed at all or is formed in a slightest amount, it will be a very great benefit.
On the other hand, the .alpha.-olefin polymer obtained from such a polymerization process as stated above contains a catalyst residue, which raises problems in various respects such as the stability and processability of the .alpha.-olefin polymer and hence necessitates a facility for removing the catalyst residue and for stabilizing the polymer.
This drawback can be offset by increasing the catalytic activity represented by the weight of .alpha.-olefin polymer produced per a unit weight of catalyst. If this catalytic activity can be increased sufficiently, the facility to remove the catalyst residue will become unnecessary and the cost required for producing the .alpha.-olefin polymer will be reducible
The present inventors previously proposed a process for producing an .alpha.-olefin polymer in the presence of a catalyst system comprising a solid catalyst component which contains at least titanium, magnesium, halogen, and electron donor, that is, a process employing a catalyst system consisting of (i) a solid obtained by reducing a titanium compound represented by the general formula Ti(OR.sup.6).sub.n X.sub.4-n, with an organomagnesium compound and treating the resulting solid with an ester compound and then with a mixture of an ether compound and titanium tetrachloride, (ii) an organoaluminum compound, and (iii) a silicic ester compound (JP-A-61-218606).
This process provides a highly stereoregular .alpha.-olefin polymer but has drawbacks in that the catalytic activity during polymerization lowers to a large extent and that the polymerization process is complicated since the catalyst system is composed of three components.
For the purpose of improving the stereoregularity of .alpha.-olefin polymers, there are disclosed processes employing each a catalyst system which comprises a third component in addition to a solid catalyst component containing titanium, magnesium, halogen, and electron donor and an organoaluminum co-catalyst, said third component comprises a sterically hindered amine having very weak reactivity on the organoaluminum (JP-A-55-127408, JP-A-58-138707, and JP-A-59-206407). According to these processes, however, the .alpha.-olefin polymer obtained still does not have a stereoregularity adequate to give satisfaction and also the catalytic activity is not sufficient to omit the operation step of removing the catalyst residue. Moreover, these processes have the drawback of being complicated in polymerization process since the catalyst system comprises three components.
On the other hand, processes for .alpha.-olefin polymerization are disclosed wherein two-component catalyst systems are used which consist each of an aluminum amide compound co-catalyst and a solid catalyst component containing titanium, magnesium, halogen, and electron donor.
According to an example of these processes, an .alpha.-olefin is polymerized in the presence of a two-component catalyst system comprising (i) a solid catalyst component composed of a transition metal halide such as TiCl.sub.4 .multidot.TiCl.sub.3 supported by stratified crystals of MgCl.sub.2 or the like and (ii) a metal amide compound represented by R'.sub.2 YNR".sub.2 wherein, R' is a C.sub.1 -C.sub.20 alkyl group, R is a bulky C.sub.5 -C.sub.20 alkyl group or forms, jointly with the neighboring nitrogen atom, a sterically hindered cyclic amino group, and Y is Al, In, or Ga (JP-A-55-127406).
According to these processes, however, the catalyst system does not have such a high activity as to permit omitting the step of removing the catalyst residue, though a high-stereoregularity .alpha.-olefin polymer can be obtained.
Under such circumstances as stated above, the problems to solve according to the present invention, in other words, an object of the present invention is to provide a process for producing an .alpha.-olefin polymer in the presence of a catalyst system which exhibits such a high activity and a stereospecificity that the removal of catalyst residue and amorphous polymer may be unnecessary.